I have this timer in my code:
Device.StartTimer(TimeSpan.FromSeconds(100), () =>
{
Device.BeginInvokeOnMainThread(() =>
{
if (detailGrid.IsVisible == true)
{
getRandomPhase();
}
});
return false;
});
Is there a way that I could (through a gesture) cancel the timer from running or abort the timer in some way?
The return value of the Timer callback is a boolean that determines if the timer should keep running or stop. You can use a private variable to keep track if the timer should keep running and return it from the callback.
Please check this sample
private bool _isTimerStart = true;
private void StartTimers()
{
try
{
Device.StartTimer(new TimeSpan(0, 0, 1), () =>
{
// do some code here
return _isTimerStart;
});
}
catch {}
} private void BtnStart_OnClicked(object sender, EventArgs e)
{
_isTimerStart = true;
StartTimers();
}
private void BtnStop_OnClicked(object sender, EventArgs e)
{
_isTimerStart = false;
}
This code was taken from the following blog post which shows a more detailed example : http://www.c-sharpcorner.com/article/quick-start-tutorial-creating-universal-apps-via-xamarin-device-classcont/
Related
How can I avoid invoking of the same event multiple times when a button is clicked rapidly.
Below is the code:
I've created a Custom Delegate Command as below
View Model
namespace TestProject.ViewModels
{
public class TestViewModel
{
public CustomDelegateCommand MenuButtonClickCommand { get; set; }
public TestViewModel()
{
MenuButtonClickCommand = new CustomDelegateCommand (async () => await ShowMenuAction());
}
private async Task ShowMenuAction()
{
//await some stuff
}
}
}
CustomDelegateCommand.cs
public class CustomDelegateTimerCommand : DelegateCommand
{
public CustomDelegateTimerCommand(Action executeMethod, Func<bool> validateMethod, Action onBusy = default(Action)) : base(executeMethod)
{
BackgroundTaskWaitHandle = new EventWaitHandle(true, EventResetMode.ManualReset);
_validateMethod = validateMethod;
_onBusy = onBusy;
}
}
The problem I'm facing is whenever a user clicks on the button rapidly, the menu list popup is opening multiple times.
I have lot of commands in my project and I need a solution that would work globally.
I tried to resolve the issue like below using ObservesCanExecute() but I don't like the idea of creating a separate variable for every command as I've a lot of commands in my project and I don't want the button to go in to disabled state when CanExecute = false.
ViewModel
MenuButtonClickCommand = new CustomDelegateCommand (async () => await ShowMenuAction().ObservesCanExecute(() => CanExecute );
private async Task ShowMenuAction()
{
CanExecute = false;
//await some stuff
CanExecute = true;
}
Any help is much appreciated!
There are 2 solutions to it. One is when you use, MVVM other is when you dont.
The non MVVM solution is delaying the execution of method for certain amount of time, like this:
public class SingleClickListener
{
private bool hasClicked;
private Action<object, EventArgs> _setOnClick;
public SingleClickListener(Action<object, EventArgs> setOnClick)
{
_setOnClick = setOnClick;
}
public void OnClick(object v, EventArgs e)
{
if (!hasClicked)
{
_setOnClick(v, e);
hasClicked = true;
}
reset();
}
private void reset()
{
Android.OS.Handler mHandler = new Android.OS.Handler();
mHandler.PostDelayed(new Action(() => { hasClicked = false; }), 500);
}
}
And then when you subscribe the onclick event:
var buttonNa = new Button { Text = "Test Button" };
buttonNa.Clicked += new SingleClickListener((sender, e) =>
{
//DO something
}).OnClick;
The Mvvm solution is bit more complicated, but its not as hacky.
TestCommand = new Command(
execute: async () =>
{
IsEditing = true;
RefreshCanExecutes();
//Fire Method
TestMethod();
},
canExecute: () =>
{
return !IsEditing;
});
public void RefreshCanExecutes()
{
(TestCommand as Command).ChangeCanExecute();
}
public void TestMethod()
{
//DO something
IsEditing = false;
RefreshCanExecutes();
}
Obviously dont forget to bind your commands to xaml :)
also second solution actually disables the button, so user cannot even tap it, first one however only ignores further taps, till time delay has finished.
Create a new class which inherits from Xamarin.Forms.Button with delay in click event, than add it to your xmal.
public class DelayedButton : Xamarin.Forms.Button
{
public DelayedButton()
{
this.Clicked += DelayedButton_Clicked;
}
async private void DelayedButton_Clicked(object sender, EventArgs e)
{
this.IsEnabled = false;
await Task.Delay(Delay);
this.IsEnabled = true;
}
public int Delay { get; set; } = 500;
}
In XAML：
<yourNameSpace:DelayedButton Delay="300" Text="DelayedButton" Command="{Binding ClickCommand}"/>
I trying to scan QR-codes in my app. It works sometimes and sometimes not. The feeling is that it doesn't work when something is loading on another thread. For example, if I press the scan button when nearby stores still are loading. Often it works if I wait.
The video capture is working because I will see the preview.
Here is my code:
private bool resultFound;
public ScannerView()
{
InitializeComponent();
On<Xamarin.Forms.PlatformConfiguration.iOS>().SetUseSafeArea(false);
view = new UIView();
ScannerArea.Children.Add(view);
}
void HandleAVRequestAccessStatus(bool accessGranted)
{
if (!accessGranted)
{
MainThread.BeginInvokeOnMainThread(async () =>
{
await NavigationHelper.Current.CloseModalAsync();
});
}
metadataOutput = new AVCaptureMetadataOutput();
var metadataDelegate = new MetadataOutputDelegate();
metadataOutput.SetDelegate(metadataDelegate, DispatchQueue.MainQueue);
session = new AVCaptureSession();
camera = AVCaptureDevice.DevicesWithMediaType(AVMediaType.Video).First();
input = AVCaptureDeviceInput.FromDevice(camera);
session.AddInput(input);
session.AddOutput(metadataOutput);
metadataOutput.MetadataObjectTypes = AVMetadataObjectType.QRCode | AVMetadataObjectType.EAN8Code | AVMetadataObjectType.EAN13Code;
metadataDelegate.MetadataFound += MetadataDelegate_MetadataFound;
camera.LockForConfiguration(out var error);
camera.VideoZoomFactor = 2;
camera.UnlockForConfiguration();
layer = new AVCaptureVideoPreviewLayer(session);
layer.VideoGravity = AVLayerVideoGravity.ResizeAspectFill;
layer.MasksToBounds = true;
layer.Frame = UIApplication.SharedApplication.KeyWindow.RootViewController.View.Bounds;
view.Layer.AddSublayer(layer);
session.StartRunning();
}
protected override void OnAppearing()
{
base.OnAppearing();
var status = AVCaptureDevice.GetAuthorizationStatus(AVMediaType.Video);
if (status != AVAuthorizationStatus.Authorized)
{
MainThread.BeginInvokeOnMainThread(() =>
{
AVCaptureDevice.RequestAccessForMediaType(AVMediaType.Video, HandleAVRequestAccessStatus);
});
return;
}
HandleAVRequestAccessStatus(true);
}
protected override void OnDisappearing()
{
base.OnDisappearing();
session.StopRunning();
session.RemoveInput(input);
session = null;
}
void Handle_Clicked(object sender, System.EventArgs e)
{
NavigationHelper.Current.CloseModalAsync();
}
void MetadataDelegate_MetadataFound(object sender, AVMetadataMachineReadableCodeObject e)
{
if (resultFound)
{
return;
}
resultFound = true;
var text = e.StringValue;
Device.BeginInvokeOnMainThread(async () =>
{
Vibration.Vibrate(100);
await NavigationHelper.Current.CloseModalAsync();
TinyPubSub.Publish(NavigationParameter.ToString(), text);
});
}
}
public class MetadataOutputDelegate : AVCaptureMetadataOutputObjectsDelegate
{
public override void DidOutputMetadataObjects(AVCaptureMetadataOutput captureOutput, AVMetadataObject[] metadataObjects, AVCaptureConnection connection)
{
foreach (var m in metadataObjects)
{
if (m is AVMetadataMachineReadableCodeObject)
{
MetadataFound(this, m as AVMetadataMachineReadableCodeObject);
}
}
}
public event EventHandler<AVMetadataMachineReadableCodeObject> MetadataFound = delegate { };
}
I have the following code:
private void CloseOrder(object sender, EventArgs e)
{
Android.Support.V7.App.AlertDialog.Builder alert = new Android.Support.V7.App.AlertDialog.Builder(this);
alert.SetTitle("Cerrar Pedido");
alert.SetMessage("Are you sure?");
alert.SetCancelable(true);
alert.SetPositiveButton("Confirm", delegate { this.Rta = true; });
alert.SetNegativeButton("Cancel", delegate { this.Rta = false; });
Dialog dialog = alert.Create();
dialog.Show();
if (this.Rta)
{
//Some code here
}
}
this.Rta is a property of my class.
The problem is that the alert doesn't show at dialog.show(), it shows once the method CloseOrder() ended, so this.Rta never gets the corresponding value assigned.
I've been searching a lot but I can't find a solution, if anyone can help me that'd be great!
dialog.Show() is asynchronous method, that means CloseOrder(object sender, EventArgs e) and dialog.Show() end up at the same time.
You can not get the 'Rta' assigned value at the CloseOrder function.
You will get the value when you click the confirm or cancel button of the dialog.
I suggest you to use message sender in the delegate{this.Rta = true}
For example:
mHandler handler = new mHandler();
Message message = new Message();
message.What = 1;
alert.SetPositiveButton("Confirm", delegate { this.Rta = true; handler.SendMessage(message); });
alert.SetNegativeButton("Cancel", delegate { this.Rta = false; handler.SendMessage(message); });
//....
class mHandler : Handler{
public override void HandleMessage(Message message) {
switch (message.What) {
case 1:
if (this.Rta)
{
//Some code here
}
break;
}
}
}
I'm developing an app and have run into a problem with asynchronous calls... Here's what i'm trying to do.
The app consumes a JSON API, and, when run, fills the ListBox within a panorama item with the necessary values (i.e. a single news article). When a user selects a ListBox item, the SelectionChanged event is fired - it picks up the articleID from the selected item, and passes it to an Update method to download the JSON response for the article, deserialize it with JSON.NET, and taking the user to the WebBrowser control which renders a html page from the response received.
The problem with this is that I have to wait for the response before I start the NavigationService, but I'm not sure how to do that properly. This way, the code runs "too fast" and I don't get my response in time to render the page.
The event code:
private void lstNews_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (lstNews.SelectedIndex == -1)
{
return;
}
ShowArticle _article = new ShowArticle();
ListBox lb = (ListBox)sender;
GetArticles item = (GetArticles)lb.SelectedItem;
string passId = ApiRepository.ApiEndpoints.GetArticleResponseByID(item.Id);
App.Current.JsonModel.JsonUri = passId;
App.Current.JsonModel.Update();
lstNews.SelectedIndex = -1;
NavigationService.Navigate(new Uri("/View.xaml?id=" + item.Id, UriKind.Relative));
}
OnNavigatedTo method in the View:
protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
base.OnNavigatedTo(e);
long sentString = long.Parse(NavigationContext.QueryString["id"]);
string articleUri = ApiRepository.ApiEndpoints.GetArticleResponseByID(Convert.ToInt32(sentString));
//this throws an error, runs "too fast"
_article = App.Current.JsonModel.ArticleItems[0];
}
The update method:
public void Update()
{
ShowArticle article = new ShowArticle();
try
{
webClient.DownloadStringCompleted += (p, q) =>
{
if (q.Error == null)
{
var deserialized = JsonConvert.DeserializeObject<ShowArticle>(q.Result);
_articleItems.Clear();
_articleItems.Add(deserialized);
}
};
}
catch (Exception ex)
{
//ignore this
}
webClient.DownloadStringAsync(new Uri(jsonUri));
}
async callback pattern:
public void Update(Action callback, Action<Exception> error)
{
webClient.DownloadStringCompleted += (p, q) =>
{
if (q.Error == null)
{
// do something
callback();
}
else
{
error(q.Error);
}
};
webClient.DownloadStringAsync(new Uri(jsonUri));
}
call:
App.Current.JsonModel.Update(() =>
{
// executes after async completion
NavigationService.Navigate(new Uri("/View.xaml?id=" + item.Id, UriKind.Relative));
},
(error) =>
{
// error handling
});
// executes just after async call above
In rx you can write :
var oe = Observable.FromEventPattern<SqlNotificationEventArgs>(sqlDep, "OnChange");
and then subscribe to the observable to convert the OnChange event on the sqlDep object into an observable.
Similarily, how can you create a Task from a C# event using the Task Parallel Library ?
EDIT: clarification
The solution pointed by Drew and then written explicitely by user375487 works for a single event. As soon as the task finished ... well it is finished.
The observable event is able to trigger again at any time. It is can be seen as an observable stream. A kind of ISourceBlock in the TPL Dataflow. But in the doc http://msdn.microsoft.com/en-us/library/hh228603(v=vs.110).aspx there is no example of ISourceBlock.
I eventually found a forum post explaining how to do that: http://social.msdn.microsoft.com/Forums/en/tpldataflow/thread/a10c4cb6-868e-41c5-b8cf-d122b514db0e
public static ISourceBlock CreateSourceBlock(
Action,Action,Action,ISourceBlock> executor)
{
var bb = new BufferBlock();
executor(t => bb.Post(t), () => bb.Complete(), e => bb.Fault(e), bb);
return bb;
}
//Remark the async delegate which defers the subscription to the hot source.
var sourceBlock = CreateSourceBlock<SomeArgs>(async (post, complete, fault, bb) =>
{
var eventHandlerToSource = (s,args) => post(args);
publisher.OnEvent += eventHandlerToSource;
bb.Complete.ContinueWith(_ => publisher.OnEvent -= eventHandlerToSource);
});
I've not tryed the above code. There may be a mismatch between the async delegate and the definition of CreateSourceBlock.
There is no direct equivalent for the Event Asynchronous Pattern (EAP) baked into the TPL. What you need to do is using a TaskCompletionSource<T> that you signal yourself in the event handler. Check out this section on MSDN for an example of what that would look like which uses WebClient::DownloadStringAsync to demonstrate the pattern.
You can use TaskCompletionSource.
public static class TaskFromEvent
{
public static Task<TArgs> Create<TArgs>(object obj, string eventName)
where TArgs : EventArgs
{
var completionSource = new TaskCompletionSource<TArgs>();
EventHandler<TArgs> handler = null;
handler = new EventHandler<TArgs>((sender, args) =>
{
completionSource.SetResult(args);
obj.GetType().GetEvent(eventName).RemoveEventHandler(obj, handler);
});
obj.GetType().GetEvent(eventName).AddEventHandler(obj, handler);
return completionSource.Task;
}
}
Example usage:
public class Publisher
{
public event EventHandler<EventArgs> Event;
public void FireEvent()
{
if (this.Event != null)
Event(this, new EventArgs());
}
}
class Program
{
static void Main(string[] args)
{
Publisher publisher = new Publisher();
var task = TaskFromEvent.Create<EventArgs>(publisher, "Event").ContinueWith(e => Console.WriteLine("The event has fired."));
publisher.FireEvent();
Console.ReadKey();
}
}
EDIT Based on your clarification, here is an example of how to achieve your goal with TPL DataFlow.
public class EventSource
{
public static ISourceBlock<TArgs> Create<TArgs>(object obj, string eventName)
where TArgs : EventArgs
{
BufferBlock<TArgs> buffer = new BufferBlock<TArgs>();
EventHandler<TArgs> handler = null;
handler = new EventHandler<TArgs>((sender, args) =>
{
buffer.Post(args);
});
buffer.Completion.ContinueWith(c =>
{
Console.WriteLine("Unsubscribed from event");
obj.GetType().GetEvent(eventName).RemoveEventHandler(obj, handler);
});
obj.GetType().GetEvent(eventName).AddEventHandler(obj, handler);
return buffer;
}
}
public class Publisher
{
public event EventHandler<EventArgs> Event;
public void FireEvent()
{
if (this.Event != null)
Event(this, new EventArgs());
}
}
class Program
{
static void Main(string[] args)
{
var publisher = new Publisher();
var source = EventSource.Create<EventArgs>(publisher, "Event");
source.LinkTo(new ActionBlock<EventArgs>(e => Console.WriteLine("New event!")));
Console.WriteLine("Type 'q' to exit");
char key = (char)0;
while (true)
{
key = Console.ReadKey().KeyChar;
Console.WriteLine();
if (key == 'q') break;
publisher.FireEvent();
}
source.Complete();
Console.ReadKey();
}
}